Fiber-reinforced plastic molded bodies having a closed cross-section are widely used in large molded bodies such as bodies and wings of aircrafts to small molded bodies such as bicycle frames, tennis rackets, fishing rods, and golf shafts. In addition, fiber-reinforced plastic molded bodies having an open cross-section are also widely used for helmets and the like.
As a core for forming a molded body having a closed cross-section, a core in which a powder-and-granular material is wrapped with a packaging film and formed into a predetermined shape by performing vacuum packaging, a core in which a large number of particle groups are accommodated in a flexible bag so as to able to be deformed into a desired shape, a core formed by blow molding, or the like is used. A core in which a vacuum-packaged powder-and-granular material is formed into a desired shape is disclosed, for example, in JP 2-238912 A (Patent Literature 1), or a core in which a large number of particle groups are accommodated in a flexible bag so as to able to be deformed into a desired shape is disclosed, for example, in JP 2012-187730 A (Patent Literature 2).
The invention described in Patent Literature 1 will be described as Conventional Example 1 of the present invention by means of FIG. 5 and FIG. 6. FIG. 5 illustrates an initial stage at which a molded article having a hollow part which is a type of closed cross-section is manufactured by a molding die 30. As illustrated in FIG. 5, a sheet-like lower fiber-reinforced thermoplastic resin material (lower FRTP) 34 which is subjected to pre-heating to be in a molten state is placed on a lower mold 31 of the molding die 30. Since the lower FRTP 34 is in the molten state, the lower FRTP 34 is hung under its own weight and enters a state of being depressed in the concave portion of the lower mold 31.
A core 33 in which a powder-and-granular material 33a is wrapped with a packaging material 33b and solidified in a predetermined shape by vacuum packaging is placed in the concave portion of the lower FRTP 34. On upper portion of the lower FRTP 34 in which the core 33 is placed, another sheet-like upper FRTP 35 which is heated to be in a molten state is placed. In this state, the circumference of the core 33 is in a state of being enclosed by the lower FRTP 34 and the upper FRTP 35.
An upper mold 32 of the molding die 30 is lowered from this state and pressurized such that the lower FITP 34 and the upper FRTP 35 are integrally molded in a state of accommodating the core 33 therein by integrating the lower FRTP 34 and the upper FRTP 35 between the upper mold 32 and the lower mold 31, thereby obtaining a molded semi-finished article. In order to discharge the core 33 from the completed molded semi-finished article, small holes are formed in the molded semi-finished article. When holes are formed in the molded semi-finished article, air infiltrates into a space between the powder-and-granular materials 33a of the vacuum-packaged core 33 and the binding of the powder-and-granular materials 33a is loosened.
Then, the powder-and-granular material 33a constituting the core 33 is discharged to the outside of the molded semi-finished article through the holes formed in the molded semi-finished article, thereby completing a molded article. If the packaging material 33b which vacuum-packages the powder-and-granular material 33a is made of a material having good releasability with respect to the molded article, the packaging material 33b can also be detached from the molded article through the holes.
Next, the invention described in Patent Literature 2 will be described as Conventional Example 2 of the present invention by means of FIG. 7. FIG. 7 illustrates a state at the time of molding a hollow molded article having a modified cross-section using a conventional core. According to this Conventional Example 2, a core 4 is used in which a particle group 4a formed from a large number of particles is wrapped with a stretchable packaging film 4b. When compression molding is carried out by lowering an upper mold 2 to press a prepreg 3 between the upper mold 2 and a lower mold 1, a mold interval holding means 20 is operated to prevent the upper mold 2 from moving to the upper side by a pair of right and left press members 21a and 21a. Simultaneously, a part of the core 4 is pressed by causing a piston rod 5a provided in the lower mold 1 to protrude into a cavity. By pressing the core 4 with the piston rod 5a, the internal pressure of the core 4 is increased to deform the core 4 while causing the particle group 4a of the core 4 to flow, thereby eliminating voids between the core 4 and the prepreg 3. There is no void inside a molded article to be obtained and a high-grade molded article is obtainable. Moreover, the mold interval holding means 20 is provided which prevents the upper mold 2 from moving in a direction apart from the lower mold 1 when the internal pressure of the core 4 is increased.
The mold interval holding means 20 has lower inclined surfaces 2a and 2a formed in upper end portions on right and left lateral surfaces (upper right and left end shoulders in FIG. 7) of the upper mold 2 and wedge faces 21b and 21 b being in slide contact with the lower inclined surfaces 2a and 2a in a surface contact state, and includes the pair of right and left press members 21a and 21a which freely slide in a direction (horizontal direction) orthogonal to a moving direction (up-and-down direction) of the upper mold 2 and a driving unit (not illustrated) which moves the press members 21a and 21a in the horizontal direction to drive them in approaching and separating directions. The shapes of the wedge faces 21b and 21b facing the pair of right and left press members 21a and 21a are set to shapes in which a space between the facing wedge faces 21b and 21 b is spread to the lower side as illustrated in FIG. 7.
Herein, as the packaging film 4b used for maintaining the shape of the core 4 used in Conventional Example 2, a nylon film, a polyethylene film, a fluororesin film, silicone, and the like can be used. Examples of preferred materials as particles having rigidity particularly include zirconia and quartz, and these materials have low thermal conductivity and granular materials having a high bending elastic modulus are obtainable therefrom.